Referring to FIG. 1, FIG. 1 is a block diagram illustrating a prior art dimmable LED driving circuit. As shown in the drawing, the driving principle thereof is mainly as follows. A voltage source 10 provides a working voltage to an LED series 20, which is coupled to a switching circuit 30, and then connected to a current detector 40 in series.
Furthermore, the current detector 40 is a resistor generally; a current value can be detected through a voltage drop that the current passes through the resistor. The current detector 40 is coupled to a comparison circuit 50, so that the voltage obtained by the current detector 40 can be compared with a triangle wave 42. When the voltage of triangle wave 42 is larger than the voltage obtained by the current detector 40, the comparison circuit 50 outputs a high-level signal; otherwise, the comparison circuit 50 outputs a low-level signal. Accordingly, a Pulse Width Modulation (PWM) signal 52 is generated for controlling connection or disconnection of the switching circuit 30. A lighting duration of the LED series 20 can be controlled through changing a duty cycle of the PWM signal 52, and a brightness of the LED series 20 can be controlled by a length of the lighting duration.
At present, there is often a light dimming demand for changing the brightness of the LED. One of the common ways is to use a variable resistor as the current detector 40. By means of changing a resistance value of the variable resistor, the voltage obtained by the current detector 40 in the same current may decrease or increase, and the duty cycle can be decreased or increased by the voltage via the comparison circuit 50, thereby changing the brightness of the LED series 20. Another way is to alter amplitude of the triangle wave 42, and the duty cycle also can be changed under condition of a constant resistance value, thereby achieving the objective of the light dimming.
However, when a switch Q2 in the switching circuit 30 disconnects instantly, a gate signal GATE, which is received by a field effect transistor Q1 in the voltage source 10 that is implemented by a boost circuit, is at a low level. Meanwhile, the LED series 20 goes out, so that the current passing through an inductor L is sharply decreased, thus resulting in a shake of a core of the inductor L to cause a larger noise. At the same time, the voltage source 10 makes large electromagnetic interference.